In an offshore drilling operation, a vessel indicated generally at 20 in FIG. 1 supports a derrick 21 at sea 22. A hollow drill string 23 suspended from the derrick 21 by a swivel connector 24, extends downwardly therefrom through a slip joint 25 and a marine riser 26 and terminates at a drill bit 27 within a wellbore 28 in the sea floor 29. The slip joint 25 comprises upper and lower cylinders 30 and 31, respectively. One end of the riser 26 is attached to a wellhead 32 embedded in the sea floor 29 and is connected to the well 28 with a typical blowout preventive (BOP) valve 33 well known in the art; the other end of the marine riser 26 is connected to the lower cylinder 31 of the slip joint 25. Riser-tensioning apparatus 34 and 35, which is also well known in the art, is attached to the upper end of the lower cylinder 31 and provides upward force necessary to support the riser 26. The upper and lower cylinders 31 and 30, respectively, of the slip joint 25 telescope to compensate for the heave of the vessel 20, i.e., motion along the z-axis caused by wave, tide, and current influences. The upper cylinder 30 strokes inside the lower cylinder 31 which remains stationery with respect to the sea floor 29 as the vessel 20 oscillates. The outer surface of the drill string 23 and the inner surfaces of the slip joint 25 and the riser 26 define the annulus 36. A conduit 37 intersects the upper portion of the upper cylinder 30 of the slip joint 25 and extends to a shale shaker 38 and active mud tanks 39. A pump 40 is connected between a standpipe 41 and the active tanks 39 from which it takes suction. The standpipe 41 is connected to a flexible hose 42 which in turn connects to the swivel connector 24 within the derrick 21.
To drill the well 28 using rotary drilling methods, drilling fluid (hereinafter referred to as "mud") is circulated by the pump 40 into the swivel 24 and through the drill string 23 to the orifices in the drill bit 27. The mud then circulates from the drill bit 27 upwardly through the annulus 36 and the conduit 37 into the shale shaker 38 where it is processed, e.g., drill cuttings removed, chemicals added, etc., and then returned to the active tanks 39 for recirculation by the pump 40. In a drilling operation, the mud has several functions, the most important being to restrain high pressure fluids 43 within various earth formations. Occasionally, the high pressure fluid 43 intrudes into the well 28 and displaces the mud. This initial intrusion is referred to as a kick. If this occurs, it is important that the pressure condition be balanced as soon as possible; otherwise, the high pressure fluid might flow up the annulus 36. This condition is known as a blowout. However, if during the drilling operation, a weak earth formation 44, is encountered, the hydrostatic pressure of the mud may fracture the rock and the mud may disburse freely into the formation 44 from the well 28. This initial fluid loss is referred to as lost returns. The condition is known as lost circulation. If the loss of mud from the annulus 36 into the formation 44 reduced the hydrostatic pressure below that of the high-pressure fluids 43, the lost circulation condition might even initiate a blowout condition.
A blowout is most effectively prevented when the kick or initial intrusion of the formation fluid is quickly detected and limited before it displaces a significant amount of mud from the well 28. Similarly, lost circulation is most effectively limited when the intiation of the loss can be quickly detected and counteracted before a significant amount of mud has flowed from the well 28 into the formation 44. Time is of the essence when trying to control these abnormal drilling conditions which may become dangerous situations. One method commonly used in the drilling industry to detect kicks or lost circulation is based on a determination of the flow of mud from the well. In this method, the output flow rate, A, i.e., the rate of flow of drilling mud returning from the well, is compared with either (i) earlier output flow rates or (ii) the input flow rate, B, i.e., the rate of mud circulating into the well. Although the former approach is commonly used, the latter approach has the advantage of compensating automatically for normal changes in the mud circulating rate by subtracting the input flow rate (B) from the output flow rate (A) to yield what is commonly referred to as the delta flow rate, D. If the sea 22 is calm, a positive delta flow rate, +D, would correspond to an equivalent increase in the output flow rate (A) and thus indicate a flow of fluid into the well 28 or a kick. Similarly, a negative delta flow rate, -D, would correspond a decrease in the return flow rate and thus indicate a flow of fluid out of the well 28 or a lost circulation.
Unfortunately, drilling offshore wells from a floating vessel complicates the monitoring of the return mud rate at sea, the heaving motion of the vessel 20 as described above, increases and decreases the output flow rate, A, which makes it impractical to measure the delta flow rate. Th maximum and minimum flow rate of the mud induced by the extension and contraction of the slip joint 25 may be several times larger or smaller than the actual or true output flow rate from the well 28. For example, variations may occur in the measured output flow rate from mud from zero gallons per minute (GPM), when the slip joint 25 is expanding, to about 1,500 GPM in the normal direction, when the slip joint 25 is contracting, compared to a true output flow rate of mud from the well 28 of about 800 GPM. The rapid determination of a blowout or lost circulation condition is impossible without a means to correct for the effects of the heave of the vessel 20 if one wishes to monitor the delta flow rate (D).
U.S. Pat. No. 3,602,322 granted August, 1971, to D. C. Gorsuch discloses a delta-flow system for determining a blowout or lost circulation on land rigs. However, its application is limited to a motionless environment because the Gorsuch system cannot effectively deal with variations in the output flow rate of mud resulting from the heaving motion of the vessel 20. Subsequent patents disclose inventions attempting to solve the heave problems by focusing on the slip joint 25 which induced time varying changes in the output flow rate (A) as discussed above. For example, U.S. Pat. No. 3,910,110 granted October, 1975, to R. K. Jeffries, et al., discloses a delta-flow system for detecting a kick or lost circulation in a subsequent well in which the output flow rate of the mud is modified by adding a coefficient corresponding to the measured volume change in the slip joint 25. U.S. Pat. No. 3,976,148 granted August, 1976, to L. D. Maus, et al., discloses a delta flow system which does not require a direct measurement of the change in volume of the slip joint 25, but rather a measurement of the fluid volume in a tightly coupled surge tank. Each of these patents have serious disadvantages such as complicated mechanical and plumbing requirements associated with the marine riser 26. Additionally, each of these patents totally disregards the other five motions of the vessel, i.e., surge, sway, roll, pitch, and yaw, all of which introduce variations in the output flow rate (A).